This invention relates to the photon conditioning of multiple gas discharge devices, especially multiple gas discharge display/memory devices which have an electrical memory and which are capable of producing a visual display or representation of data such as numerals, letters, radar displays, aircraft displays, binary words, educational displays, etc.
Multiple gas discharge display and/or memory panels of one particular type with which the present invention is concerned are characterized by an ionizable gaseous medium, usually a mixture of at least two gases at an appropriate gas pressure, in a thin gas chamber or space between a pair of opposed dielectric charge storage members which are backed by conductor (electrode) members, the conductor members backing each dielectric member typically being appropriately oriented so as to define a plurality of discrete gas discharge units or cells.
In some prior art panels the discharge cells are additionally defined by surrounding or confining physical structure such as apertures in perforated glass plates and the like so as to be physically isolated relative to other cells. In either case, with or without the confining physical structure, charges (electrons, ions) produced upon ionization of the elemental gas volume of a selected discharge cell, when proper alternating operating potentials are applied to selected conductors thereof, are collected upon the surfaces of the dielectric at specifically defined locations and constitute an electrical field opposing the electrical field which created them so as to terminate the discharge for the remainder of the half cycle and aid in the initiation of a discharge on a succeeding opposite half cycle of applied voltage, such charges as are stored constituting an electrical memory.
Thus, the dielectric layers prevent the passage of substantial conductive current from the conductor members to the gaseous medium and also serve as collecting surfaces for ionized gaseous medium charges (electrons, ions) during the alternate half cycles of the A.C. operating potentials, such charges collecting first on one elemental or discrete dielectric surface area and then on an opposing elemental or discrete dielectric surface area on alternate half cycles to constitute an electrical memory.
An example of a panel structure containing nonphysically-isolated or open discharge cells is disclosed in U.S. Pat. No. 3,499,167 issued to Theodore C. Baker, et al.
An example of a panel containing physically isolated cells is disclosed in the article by D. L. Bitzer and H. G. Slottow entitled "The Plasma Display Panel -- A Digitally Addressable Display With Inherent Memory," Proceeding of the Fall Joint Computer Conference, IEEE, San Francisco, Cal., Nov. 1966, pp. 541-547. Also reference is made to U.S. Pat. No. 3,559,190.
In the construction of the panel, a continuous volume of ionizable gas is confined between a pair of dielectric surfaces backed by conductor arrays typically forming matrix elements. The cross conductor arrays may be orthogonally related (but any other configuration of conductor arrays may be used) to define a plurality of opposed pairs of charge storage areas on the surfaces of the dielectric bounding or confining the gas. Thus, for a conductor matrix having H rows and C columns the number of elemental discharge cells will be the product H .times. C and the number of elemental or discrete areas will be twice the number of such elemental discharge cells.
In addition, the panel may comprise a so-called monolithic structure in which the conductor arrays are created on a single substrate and wherein two or more arrays are separated from each other and from the gaseous medium by at least one insulating member. In such a device the gas discharge takes place not between two opposing electrodes, but between two contiguous or adjacent electrodes on the same substrate; the gas being confined between the substrate and an outer retaining wall.
It is also feasible to have a gas discharge device wherein some of the conductive or electrode members are in direct contact with the gaseous medium and the remaining electrode members are appropriately insulated from such gas, i.e., at least one insulated electrode.
In addition to the matrix configuration, the conductor arrays may be shaped otherwise. Accordingly, while the preferred conductor arrangement is of the crossed grid type as discussed herein, it is likewise apparent that where a maximal variety of two dimensional display patterns is not necessary, as where specific standardized visual shapes (e.g., numerals, letters, words, etc.) are to be formed and image resolution is not critical, the conductors may be shaped accordingly, i.e., a segmented display.
The gas is one which produces visible light or invisible radiation which stimulates a phosphor (if visual display is an objective) and a copious supply of charges (ions and electrons) during discharge.
In prior art, a wide variety of gases and gas mixtures have been utilized as the gaseous medium in a gas discharge device. Typical of such gases include CO; CO.sub.2 ; halogens; nitrogen; NH.sub.3 ; oxygen; water vapor; hydrogen; hydrocarbons; P.sub.2 O.sub.5 ; boron fluoride, acid fumes; TiCl.sub.4 ; Group VIII gases; air; H.sub.2 O.sub.2 ; vapors of sodium, mercury, thallium, cadmium, rubidium, and cesium; carbon disulfide; laughing gas; H.sub.2 S; deoxygenated air; phosphorus vapors; C.sub.2 H.sub.2 ; CH.sub.4 ; naphthalene vapor; anthracene; freon; ethyl alcohol; methylene bromide; heavy hydrogen; electron attaching gases; sulfur hexafluoride; tritium; radioactive gases; and the rare or inert gases.
In one preferred embodiment hereof the medium comprises at least one rare gas, more preferably at least two, selected from helium, neon, argon, krypton, or xenon.
In an open cell Baker, et al. type panel, the gas pressure and the electric field are sufficient to laterally confine charges generated on discharge within elemental or discrete dielectric areas within the perimeter of such areas, especially in a panel containing non-isolated discharge cells. As described in the Baker, et al. patent, the space between the dielectric surfaces occupied by the gas is such as to permit photons generated on discharge in a selected discrete or elemental volume of gas to pass freely through the gas space and strike surface areas of dielectric remote from the selected discrete volumes, such remote, photon struck dielectric surface areas thereby emitting electrons so as to condition at least one elemental volume other than the elemental volume in which the photons originated.
With respect to the memory function of a given discharge panel, the allowable distance or spacing between the dielectric surfaces depends, inter alia, on the frequency of the alternating current supply, the distance typically being greater for lower frequencies.
While the prior art does disclose gaseous discharge devices having externally positioned electrodes for initiating a gaseous discharge, sometimes called "electrodeless discharge," such prior art devices utilized frequencies and spacing or discharge volumes and operating pressures such that although discharges are initiated in the gaseous medium, such discharges are ineffective or not utilized for charge generation and storage at higher frequencies; although charge storage may be realized at lower frequencies, such charge storage has not been utilized in a display/memory device in the manner of the Bitzer-Slottow or Baker, et al. invention.
The term "memory margin" is defined herein as ##EQU1## where V.sub.f is the half amplitude of the smallest sustaining voltage signal which results in a discharge every half cycle, but at which the cell is not bi-stable and V.sub.E is the half amplitude of the minimum applied voltage sufficient to sustain discharges once initiated.
It will be understood that the basic electrical phenomenon utilized in this invention is the generation of charges (ions and electrons) alternately storable at pairs of opposed or facing discrete points or areas on a pair of dielectric surfaces backed by conductors connected to a source of operating potential. Such stored charges result in an electrical field opposing the field produced by the applied potential that created them and hence operate to terminate ionization in the elemental gas volume between opposed or facing discrete points or areas of dielectric surface. The term "sustain a discharge" means producing a sequence of momentary discharges, at least one discharge for each half cycle of applied alternating sustaining voltage, once the elemental gas volume has been fired, to maintain alternate storing of charges at pairs of opposed discrete areas on the dielectric surfaces.
As used herein, a cell is in the "on state" when a quantity of charge is stored in the cell such that on each half cycle of the sustaining voltage, a gaseous discharge is produced.
In addition to the sustaining voltage, other voltages may be utilized to operate the panel, such as firing, addressing, and writing voltages.
A "firing voltage" is any voltage, regardless of source, required to discharge a cell. Such voltage may be completely external in origin or may be comprised of internal cell wall voltage in combination with externally originated voltages.
An "addressing voltage" is a voltage produced on the panel X - Y electrode coordinates such that at the selected cell or cells, the total voltage applied across the cell is equal to or greater than the firing voltage whereby the cell is discharged.
A "writing voltage" or "write voltage" is an addressing voltage of sufficient magnitude to make it probable that on subsequent sustaining voltage half cycles, the cell will be in the "on" state.
It must be explained that it is possible to have a write voltage V.sub..omega. which is not large enough in amplitude to ensure transferring a cell to the on state in 100% of a series of trials. We define V.sub..sub..omega.100 as the minimum write voltage amplitude sufficient to guarantee near 100% success. The qualifier "near" is necessary because the initiation of a gas discharge is a statistical process, which can be very certain, but never absolutely certain.
It is well known that the initiation of a gas discharge requires not only the application of a voltage across the gas, but also the presence of "starting electrons," which can be accelerated by the voltage. A common method of providing these electrons is to operate in the vicinity of the gas discharge to be initiated, one or more other gas discharges, whose function is to emit photons which, upon striking material surfaces in the vicinity of the cell site where a discharge is to be initiated, will photoelectrically create starting electrons. We refer to this method as "photon conditioning."
If the rate of production of starting electrons is low, it is easy to observe a so-called "statistical lag" -- a period after the application of voltage before enough starting electrons have appeared to initiate the discharge. This lag can be reduced either by increasing the applied voltage (thereby increasing the likelihood that any particular starting electron will initiate a discharge) or by increasing the supply of starting electrons. (See for example G. F. Weston, Cold Cathode Glow Discharge Tubes, London ILIFFE Books Ltd., 1968. FIG. 5.2, page 156.)
It will be clear that if a discharge is to be initiated reliably with a brief pulse of applied voltage, the statistical lag must be made substantially less than the pulse width. This effect may be achieved either by using a high-voltage pulse or by providing a copious supply of starting electrons. The practice of this invention comprises the providing of a copious supply of starting electrons by means of saturated photon conditioning so as to improve the performance of a gas discharge device.
More particularly, there is disclosed the saturated photon conditioning of a multiple gas discharge device by the provision of a sufficient flux of conditioning photons at each to-be-conditioned cell such that the necessary writing pulse amplitude for each cell is reduced to a minimum level.
Still more particularly, in accordance with this invention, there is provided a conditioning photon flux sufficient to reduce the necessary write voltage of the cell to such a level that the provision of more conditioning photons would not significantly further reduce the amplitude of the required writing pulse.
It has been discovered that the utilization of such saturated photon conditioning has several advantages including the tendency to improve the uniformity of panel operation and also the tendency to decrease the required sustaining voltage. Other advantages may also result.
In the practice of this invention, saturated photon conditioning is measured by observing the minimum write voltage required to write one or more selected cells of the panel. Typically, the selection is made at or near the center of the panel matrix since the center cells are usually the most difficult to condition and to write.
After such cells are selected, the panel is subjected to conditioning light intensity sufficient to achieve saturated photon conditioning.
The conditioning light intensity (photon flux) may be increased by any one or more of several means:
a. increasing the driving voltage on the conditioning discharge or discharges; PA1 b. increasing the area of the conditioning discharge or discharges; PA1 c. varying the gas mixture; PA1 d. optimizing the timing of a pulse of conditioning light such that the conditioning photon flux peaks in intensity at the start of a write pulse so that the photoelectrons will be maximally useful in initiating the discharge. PA1 1. The voltage necessary to address any desired site in the device is minimized. PA1 2. The elimination of non-uniformities in operation of different sites in the device which may be attributable to non-uniformity of conditioning. PA1 3. In a matrix-addressed device, wherein many nonaddressed discharge cell sites see half the voltage which appears at the addressed site, the invention reduces the danger that a well-conditioned cell may fire on 1/2 V.sub..omega. where V.sub..omega. is the voltage required to fire an ill-conditioned site.
Alternatively, the "effective light intensity" may be increased by varying either gas composition or the surface composition and processing so as to provide a more optimum match between the photon wavelengths and the photoemissive yield curve of the surface; thus a given photon flux will provide a larger number of starting electrons.
In any event, this invention requires that, by any available means, the conditioning light intensity or photon flux be made high enough so as to approach saturation at the worst-conditioned discharge cell site in the panel. Then all other discharge sites will exhibit saturated or near-saturated conditioning as well. The advantages of this invention are: